In the manufacturing of a variety of electronic devices, such as semiconductor devices and liquid crystal devices, having microstructures, a lithography method is widely used, and as the device structures are miniaturized, resist patterns in the lithography process are desired to be miniaturized.
Currently, in a state-of-the-art lithography technique, for example, a fine resist pattern having a line width as small as 90 nm can be formed by a lithography method, and the formation of a further finer pattern will be needed in the future.
For achieving the formation of such a fine pattern that is smaller than 90 nm, the development of an improved exposure system and the corresponding resist is the most important. The improvement of the exposure system generally consists of the development of a light source, such as an F2 laser, an EUV (extreme ultraviolet) light, an electron beam, or an X-ray, having a shorter wavelength and the development of a lens having an increased numerical aperture (NA).
However, the light source having a shorter wavelength has a problem in that it requires a new expensive exposure system, and the increase of NA has a problem in that there is a trade-off between the resolution and the depth of focus and the increase of the resolution lowers the depth of focus.
Recently, as a lithography technique which can solve the problems, a liquid immersion lithography method has been reported (For example, Non-patent document 1 (Journal of Vacuum Science & Technology B (J. Vac. Sci. Technol. B) (Published in the United States) 1999, Vol. 17, 6th number, pp. 3306-3309), Non-patent document 2 (Journal of Vacuum Science & Technology B (J. Vac. Sci. Technol. B) (Published in the United States) 2001, Vol. 19, No. 6, pp. 2353-2356), and Non-patent document 3 (Proceedings of SPIE Vol. 4691 (Published in the United States) 2002, Vol. 4691, pp. 459-465)). This method is such that a resist film is exposed through a liquid refractive index medium (immersion liquid), such as pure water or a fluorine-based inert liquid, having a predetermined thickness at least on the resist film between a lens and the resist film on a substrate. In this method, the space of the path of exposure light, which has conventionally been filled with inert gas, such as air or nitrogen gas, is replaced by a liquid having a larger refractive index (n), for example, pure water, and therefore, the use of a light source having a wavelength for the exposure conventionally used can achieve high resolution without lowering the depth of focus like the use of a light source having a shorter wavelength or a lens having a higher NA.
By employing the liquid immersion lithography, a resist pattern having higher resolution and excellent depth of focus can be formed at a low cost using a lens mounted on the existing exposure system, and hence the liquid immersion lithography has attracted considerable attention.